/**
 * Array.prototype.entries()方法返回一个Array Iterator对象，该对象包含数组中每一个索引的键值对
 */
var arr = ["a", "b", "c"].entries();
console.log(arr.next())//{ value: [ 0, 'a' ], done: false }
console.log(arr.next())//{ value: [ 1, 'b' ], done: false }
console.log(arr.next())//{ value: [ 2, 'c' ], done: false }
console.log(arr.next())//{ value:undefined, done: true }